The TFTP server accepts a DATA datagram of any size. The dispatcher rejects datagrams shorter than
four bytes (nxd_tftp_server.c:1037) and nothing anywhere checks an upper bound, in particular not
against the protocol maximum of 4 + NX_TFTP_FILE_TRANSFER_MAX. Two things follow from that one
missing check, both reachable before any authentication because TFTP has none.
The handler passes `nx_packet_length – 4` straight to FileX:
“`c
/* addons/tftp/nxd_tftp_server.c:1863, 1889 */
status = nx_packet_copy(packet_ptr, &temp_ptr,
server_ptr -> nx_tftp_server_packet_pool_ptr, NX_WAIT_FOREVER);
…
fx_file_write(&(client_request_ptr -> nx_tftp_client_request_file),
packet_ptr -> nx_packet_prepend_ptr + 4,
packet_ptr -> nx_packet_length – 4);
“`
`nx_packet_length` is the length of a chain, not of one contiguous buffer, so FileX copies past the
end of the first packet:
“`
ERROR: AddressSanitizer: heap-buffer-overflow
READ of size 1280 at 0x621000001108 thread T5
#0 __interceptor_memcpy
#1 _fx_utility_memory_copy filex/common/src/fx_utility_memory_copy.c:78
0x621000001108 is 0 bytes to the right of 4104-byte region
“`
Those bytes are written into the file the attacker is uploading, and a TFTP read request hands them
back, so this is a memory disclosure with a convenient retrieval channel.
The same datagram also wedges the server. `nx_packet_copy` at :1863 needs
ceil(nx_packet_length / pool_payload) packets and asks for them with NX_WAIT_FOREVER, so when the
attacker sizes the datagram beyond what the pool holds, the server thread suspends and never
returns. A liveness probe after one such datagram times out with the pool at 0 of 12 packets and
the server thread suspended, and no later client is served.
Reject `nx_packet_length > 4 + NX_TFTP_FILE_TRANSFER_MAX` in the DATA branch before either call,
and use a bounded wait rather than NX_WAIT_FOREVER for the copy.
